STUDIES IN SPICULE FORMATION. 289 
spindle-shaped cells.” Bourne mistook these strands of 
endoderm cells for scleroblasts—“ the scleroblasts have the 
form of irregularly polygonal, ovate, or amcebiform cells, 
varying very much in size and shape; they run in strands 
and patches through the mesoglcea at the bases of the expanded 
polyps, and may be found, though they are not easily studied, 
in the thickened mesoglcea of the cenenchyme” (6)-—but the 
endoderm cells as a whole, besides often being multi-nucleated 
(which the scleroblasts never are until spicule-formation is 
fairly advanced, and then only two nuclei are present) are 
also considerably larger. and much more irregular in shape 
than the true scleroblasts, which are approximately spherical. 
It is true that in many cases these endoderm cells possess 
large vacuoles, which, but for the absence of refringency, 
might be mistaken, at first glance, for young spicules, but, 
personally, I cannot call to mind having ever seen a real 
spicule contained in one of these irregular cells. 
Bourne describes, in the paper before referred to, two 
other kinds of cells—one possessing refringent granules and 
the other containing the ovoid bodies—only the latter of 
which Hickson refers to. Bourne remarks that the cells of 
the first kind are “rather smaller than, but of similar shape 
to, the scleroblasts,” which, since Bourne reckons the endo- 
derm cells as scleroblasts, must therefore be somewhat irregu- 
lar in form, as his figures indicate. These cells are said to be 
‘filled with minute highly-refringent granules,” and their 
nucleus is “ rarely to be seen, being hidden by the granules.” 
Bourne believes that “ their function is to secrete the gelatinoid 
substance of the mesoglea.” ‘To some extent I can confirm 
Bourne’s statements, since I also have found peculiar cells 
containing very distinct minute granules and a faintly-staining 
nucleus, and which are “ of similar shape to the scleroblasts.” 
The cells I recognise as scleroblasts are, however, spherical, 
and so are these cells containing the distinct granules; 
whether they are the same as the jelly-secreting cells of 
Bourne I cannot decide. I have shown three of these cells, 
which are somewhat bladder-like in appearance, in fig. 21. 
